BERKELEY, CA – Sometimes, it pays
to think small. By observing how a single electron behaves amid a cluster
of water molecules, a team of scientists has gained a better understanding
of a fundamental process that drives a myriad of biological and chemical
phenomena, such as the formation of reactive molecules in the body that
can cause disease.

Neumark

The researchers, led by Lawrence Berkeley National Laboratory’s
Chemical Sciences Division Director Daniel Neumark, used an extremely
fast imaging technique to observe an excited electron, surrounded by several
dozen water molecules, relax back to its original energy state. This journey
occurred much more quickly than one theory predicts, lending credence
to an opposing theory and helping to solve a longstanding puzzle in the
world of hydrated electrons.

“Our work tells us something very basic about the nature of the
interactions between electrons and water, which is of general, cross-cutting
interest to many scientists,” says Neumark, who conducted the study
with scientists from the University of California at Berkeley and Israel’s
Tel-Aviv University. Their research is published in the September 16,
2004 edition of Science Express.

As their name implies, hydrated electrons are electrons that are dissolved
in water. They occupy an elliptical void formed by six water molecules,
and they’ve intrigued scientists since their discovery in 1962.
The simple fact that they exist is interesting, as is their little understood
role in many biological and chemical processes. Although it is too early
to tell how Neumark’s work will elucidate the behavior of hydrated
electrons in the real world, such as how they conspire to form free radicals
(highly reactive molecules that can damage tissue and contribute to diseases
such as cancer, rheumatoid arthritis, and heart disease), it will help
shape future research.

Leading up to this study, scientists had been divided as to how hydrated
electrons react after they’ve been excited. One theory holds that
electrons convert back to their original energy state in about 50 femtoseconds,
or 50 millionths of a billionth of a second. The other theory contends
this conversion takes much longer, about 500 femtoseconds.

Most research into this phenomenon has explored the behavior of hydrated
electrons in a large quantity of water, called a bulk. Bulk experiments
can yield very precise measurements, but they have trouble portraying
the various components of the electrons’ journey between energy
states. To get a more precise look, Neumark’s team instead observed
a single electron in a tiny cluster of between 25 and 50 water molecules.
Such clusters give scientists an extremely close look at the electron’s
dynamics. For example, they can determine whether water molecules are
simply rearranging themselves around an electron in an excited or a ground
state, or whether these dynamics indicate the actual transition of the
electron between these states.

The team created an electronic excitation by zapping the cluster with
a femtosecond laser pulse. They then used time-resolved photoelectron
imaging to take snapshots of the electron as it relaxed back to its ground
state. The dynamics and rate of this conversion, when extrapolated to
how hydrated electrons behave in bulk, suggest that hydrated electrons
relax back to their unexcited state in about 50 femtoseconds — a
finding that tips the scales in favor of this theory.

“Resolving which of these two models is correct is a key step.
We’ve used time-resolved studies of finite clusters to resolve an
issue of fundamental importance, namely the dynamics of an excited hydrated
electron,” says Neumark. “More generally, this work represents
a fairly unique example of how studies of clusters can elucidate bulk
phenomena.”

This research is supported in part by the National Science Foundation.

Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California. Visit our website at www.lbl.gov/.